A silo-type combustor usually comprises a combustion chamber, which is supplied with a fuel from above and compressed air, and a hot gas casing through which the hot combustion products are supplied from the combustion chamber to a turbine. The hot gases expand through the turbine.
The compressed air is supplied to the combustion chamber through an annular channel, which surrounds the combustor casing. The flow of hot gases and the flow of compressed air are therefore in opposing directions. The compressed air has further the function of cooling the combustor chamber casing and the hot gas casing.
The lower part of the combustion chamber is called the combustor inner liner (CIL). The combustor inner liner and the hot gas casing have an overlap, where the tip of the combustor inner liner is located inside of the upper part of the hot gas casing. In order to avoid mechanical contact and resulting fretting a gap is provided between the combustor inner liner and the hot gas casing. As the compressed air is at a higher pressure than the hot gases flowing through the combustor inner liner and the hot gas casing, the pressure difference would cause leakage of compressed air from the annular channel into the hot gas flow, thus reducing the amount of air available for combustion. A seal is therefore provided in the gap to prevent any leakage.
The seal consists of a set of segments, which may or may not overlap. One seal design is called a belt seal. In this seal arrangement the segments are assembled circumferentially around the casing, whereby the individual segments are assembled with overlap and tightened by a spring. Their connection allows some movement relative to each other, and therefore they form a flexible structure, which accommodates the possible deformation of the combustor inner liner and the hot gas casing due to thermal expansion. Hooks welded onto the hot gas casing hold the belt seal segments. The interfaces between the seal segments are circumferentially located on the hot gas casing at the same positions as the hooks.
The belt seal structure accommodates a deformation of the combustor casing due to thermal expansion and a resulting variation in the radial gap between the combustor inner liner and the hot gas casing. The overlapping sections of the belt seal substantially prevent leakage past the seal in the axial direction. There are however still small gaps between the segments, which lead to leakages in the radial direction. The amount of leakage air is small and has no appreciable impact on the combustion air supply. These leakages can however, due to vortex formation, cause ingestion of hot gas into the gap between the combustor inner liner and the hot gas casing. This results in local overheating. This overheating may occur on the combustor inner liner as well as on the hot gas casing, in the same position where the holding hooks and the interface between the segments is located. These hot spots lead to increased oxidation and a reduced life of the combustor inner liner and of the hot gas casing.